Mice born with combined lipase deficiency (cld/cld) have very low levels of lipoprotein and hepatic lipase activities, develop severe hypertriglyceridemia, and die within 3 days. The recessive mutation (cld) causing the deficiency is located on chromosome 17, whereas structural genes for these lipases are located on chromosomes 8 and 11, respectively. These lipases, both glycoproteins, are synthesized in parenchymal cells and transferred to endothelial cells where they act. Southern blot analyses of endonuclease-digests of the lipoprotein lipase gene and RNase A protection assays of hepatic lipase mRNA suggest that the structural genes for the two lipases are normal in cld/cld mice. Lipoprotein lipase synthesized in cld/cld brown adipocytes, unlike that in unaffected cells, is inactive, endo H-sensitive, and retained in endoplasmic reticulum. Lipoprotein lipase of normal cells is found in both aqueous and Triton X-114 extracts of cells, whereas lipase 6f cld/cld cells is found only in the aqueous extract, suggesting defective binding of the lipase to membranes, possibly endoplasmic reticulum, in cld/cld cells. Hepatic lipase in liver of cld/cld mice, although inactive, is secreted to the extracellular space. These findings suggest that the cld mutation affects the two lipases in different ways. Unaffected mouse brown adipocytes synthesize lipoprotein lipase which is active, has endo H-resistant (complex) oligosaccharides, and is secreted. Cells treated with tunicamycin produce unglycosylated lipoprotein lipase which is inactive and retained in endoplasmic reticulum. Blocking the action of Golgi mannosidase I with 1-deoxymannojirimycin, or mannosidase II with swainsonine, causes formation of lipase which is totally endo H-sensitive, yet active and secreted. Monensin treatment also blocks processing of lipoprotein lipase to the endo H-resistant form without affecting the synthesis of active lipase. Secretion of lipase, however, is impaired because of the direct blocking action of monensin on transport between cis and trans Golgi. The findings indicate that lipoprotein lipase becomes active (dimeric) and secretable during some process(es) between core-glycosylation in endoplasmic reticulum and trimming by mannosidases in Golgi. The importance of processing lipoprotein lipase to the endo H-resistant form is not yet known.